To remove B from Si, as described in Suzuki et al., Journal of the Japan Institute of Metals, vol. 54, no. 2, p. 168 to 172 (1990), there is the method of bringing molten Si into contact with CaO—MgO—SiO2, CaO—BaO—SiO2, CaO—CaF2—SiO2, or other slag to make the B move to the slag by the equilibrium distribution of B between the slag and molten Si.
Further, Japanese Patent Publication (A) No. 8-11208 discloses a method of refining Si comprising treating molten Si by slag so as to remove impurities from the molten Si, which method adds said slag to molten Si continuously or substantially continuously and, at the time when the slag and molten Si are in an equilibrium state for one or more types of impurity elements, deactivates said slag continuously or substantially continuously or removes it from the molten Si.